It was nearing closing time in March last year when a manager at Boffi Georgetown dispatched a series of alarmed messages. Observing two men yelling outside the luxury kitchen and bath showroom, Julia Walter reached for her phone and accessed a private messaging application that hundreds of residents, retailers and police in this overwhelmingly white, wealthy neighborhood use to discuss people they deem suspicious.

“2 black males screaming at each other in alley,” Walter wrote. “. . . Help needed.”

One minute later, a District police officer posted he would check it out, and Walter felt relieved. But as weeks gave way to months and the private group spawned hundreds of messages, Walter’s relief turned to unease. The overwhelming majority of the people the app’s users cited were black. Was the chatroom reducing crime along the high-end retail strip? Was it making people feel safer? Or was it racial profiling?